The present invention relates to a system for controlling the air-fuel ratio for an automotive engine having a fuel injection system.
In an electronic fuel-injection control system, the amount of fuel to be injected into the engine is determined in accordance with engine operating variables such as mass air flow, engine speed and engine load. The amount of fuel is determined by a fuel injector energization time (injection pulse width). Basic injection pulse width (T.sub.p) can be obtained by the following formula. EQU T.sub.p =k.times.Q/N (1)
where Q is mass airflow, N is engine speed, and K is a constant.
Desired injection pulse width (T.sub.i) is obtained by correcting the basic injection pulse width (T.sub.p) with coefficients for engine operating conditions, variables, and other factors. The following is an example of a formula for computing the desired injection pulse width. EQU T.sub.i =T.sub.p .times..alpha..times.K.sub.TW .times.K.sub.MR .times.K.sub.OT ( 2)
where .alpha. is a correcting coefficient for the output of an O.sub.2 -sensor provided in an exhaust passage, and K.sub.TW is a correcting coefficient for coolant temperature, K.sub.MR is a correcting coefficient for driving conditions, and K.sub.OT is a coefficient for other variables.
The coefficient K.sub.MR is provided for correcting the deviation of the air-fuel ratio from a desired ratio, which is caused by characteristics of fuel injectors. The coefficient K.sub.MR is stored in a three-dimensional table having an axis of calculated fuel injection pulse width (fuel injection quantity) and another axis of engine speed, divided in each address. Accordingly, the fuel injection width (T.sub.p) is corrected by the coefficient K.sub.MR stored in the table in accordance with driving conditions.
The fuel injection pulse width (fuel injection quantity) calculated by the formula (2) increases with an increase of intake mass airflow. In a range where the fuel injection quantity is small, the fuel injection pulse width can be sufficiently corrected by the coefficient K.sub.MR. Meanwhile, when the fuel injection quantity increases, the flow-back of the intake occurs. During closing of an intake valve of the engine. The amount of the flow-back air increases with an increase of the opening degree of the throttle valve of the engine. If a mass airflow sensor with a hot wire device is used as a mass airflow sensor, the sensor will operate to sense the flow-back mass airflow as new intake mass airflow at closing of an intake valve. The amount of the flow-back mass airflow increases with an increase of the opening degree of the throttle valve of the engine. Accordingly, the airflow sensor generates a signal representing a large mass airflow in spite of actually a small mass airflow. As a result, the calculated fuel injection pulse width is improperly increased, which causes excessive enrichment of the fuel mixture. Such a deviation of the air-fuel ratio is increased when the vehicle is driven at a high altitude, since the throttle valve is widely opened compared with a low altitude.